feat(stdlib): Time/Date/Duration calendar-clock core (issue #9)

Implement the calendar/clock half of #9 as plain-i32 integer epochs — no
new type, no floating point (the issue's "integer epochs to avoid drift") —
so every operation is deterministic and bit-identical on every platform:

  Duration — a span in whole seconds; seconds/minutes/hours/days build one,
             as_seconds/as_minutes/as_hours/as_days read it back. Because a
             duration is just an int, `+` and `>` work with no extra machinery
             (Duration.minutes(5) + Duration.seconds(30), away > Duration.hours(3)).
  Date     — a civil day as days-since-1970 (UTC): new/year/month/day/weekday/
             is_leap/days_in_month/to_epoch/add_days/diff_days.
  DateTime — an instant as seconds-since-1970 (UTC, matching Time.now):
             from/date/add/year/month/day/weekday/hour/minute/second.
  Time.since(past) = now - past, for offline-progress / "time away" checks.

New selfhost/emit_datetime.ludic (is_/emit_ for the three namespaces, wired
into emit_ns_call + the frag list). The two civil<->epoch conversions are
Howard Hinnant's public-domain proleptic-Gregorian algorithms, emitted once
per program as the @fn_days_from_civil / @fn_civil_from_days prelude and gated
by g_uses_datert; days_in_month is next-month-day-0 (no lookup table). Time
gains `since`. Docs (Duration/Date/DateTime sections, 28 method pages +
time-since), inventory, and LSP hover kept in sync; a registered test checks
component math against hand-computed values. Reseeded; C-free fixpoint holds;
all suites green (26 self-host / 45 regression / 29 tools); check.py,
check-impl.py and validate.py OK.

format/parse, a game-controlled simulated clock, and timezones are tracked
follow-ups; v1 is UTC-only and, on the i32 epoch, valid through 2038.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/datetime/_section.md

@@ -0,0 +1,7 @@
+---
+id: datetime
+title: DateTime
+order: 6
+---
+
+Instants on the wall-clock timeline (UTC), stored as a plain <code>int</code>: the count of seconds since 1970-01-01, matching <code>Time.now</code>. Build one with <code>DateTime.from</code>, shift it by a <code>Duration</code> with <code>DateTime.add</code>, and pull it apart with the <code>year</code>/<code>month</code>/<code>day</code>/<code>hour</code>/<code>minute</code>/<code>second</code> readers. Deterministic integer math throughout; v1 is UTC-only with no leap seconds, and assumes instants at or after 1970.

docs/language/datetime/datetime-add.md

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+---
+id: datetime-add
+name: DateTime.add
+category: datetime
+kind: namespace-method
+tokens: DateTime.add
+sig: DateTime.add(dt, span) -> int
+tip: The instant `span` seconds later.
+order: 2
+ns: DateTime
+member: add
+---
+
+The instant <code>span</code> seconds after <code>dt</code>. Pair it with the <code>Duration</code> constructors to express the offset in real units.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let ready_at = DateTime.add(Time.now(), Duration.hours(3))
+  }
+}
+```

docs/language/datetime/datetime-date.md

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+---
+id: datetime-date
+name: DateTime.date
+category: datetime
+kind: namespace-method
+tokens: DateTime.date
+sig: DateTime.date(dt) -> int
+tip: The calendar day the instant falls on.
+order: 1
+ns: DateTime
+member: date
+---
+
+The <code>Date</code> (days since 1970) that the instant <code>dt</code> falls on, dropping the time of day.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let today = DateTime.date(Time.now())
+  }
+}
+```

docs/language/datetime/datetime-day.md

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+---
+id: datetime-day
+name: DateTime.day
+category: datetime
+kind: namespace-method
+tokens: DateTime.day
+sig: DateTime.day(dt) -> int
+tip: The day of the month (1-31) of an instant.
+order: 5
+ns: DateTime
+member: day
+---
+
+The day of the month of the instant <code>dt</code> (UTC), <code>1</code>&ndash;<code>31</code>.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let d = DateTime.day(DateTime.from(2000, 3, 9, 0, 0, 0))   # 9
+  }
+}
+```

docs/language/datetime/datetime-from.md

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+---
+id: datetime-from
+name: DateTime.from
+category: datetime
+kind: namespace-method
+tokens: DateTime.from
+sig: DateTime.from(year, month, day, hour, minute, second) -> int
+tip: Build an instant from its calendar parts.
+order: 0
+ns: DateTime
+member: from
+---
+
+Builds a <code>DateTime</code> (seconds since 1970, UTC) from calendar and clock parts. The inverse of the component readers below.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let t = DateTime.from(2000, 1, 1, 12, 30, 15)
+  }
+}
+```

docs/language/datetime/datetime-hour.md

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+---
+id: datetime-hour
+name: DateTime.hour
+category: datetime
+kind: namespace-method
+tokens: DateTime.hour
+sig: DateTime.hour(dt) -> int
+tip: The hour of day (0-23) of an instant.
+order: 7
+ns: DateTime
+member: hour
+---
+
+The hour of the day of the instant <code>dt</code> (UTC), <code>0</code>&ndash;<code>23</code>. Drives day/night cycles from a real clock.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let h = DateTime.hour(DateTime.from(2000, 1, 1, 12, 30, 15))   # 12
+  }
+}
+```

docs/language/datetime/datetime-minute.md

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+---
+id: datetime-minute
+name: DateTime.minute
+category: datetime
+kind: namespace-method
+tokens: DateTime.minute
+sig: DateTime.minute(dt) -> int
+tip: The minute of the hour (0-59) of an instant.
+order: 8
+ns: DateTime
+member: minute
+---
+
+The minute within the hour of the instant <code>dt</code> (UTC), <code>0</code>&ndash;<code>59</code>.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let mi = DateTime.minute(DateTime.from(2000, 1, 1, 12, 30, 15))   # 30
+  }
+}
+```

docs/language/datetime/datetime-month.md

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+---
+id: datetime-month
+name: DateTime.month
+category: datetime
+kind: namespace-method
+tokens: DateTime.month
+sig: DateTime.month(dt) -> int
+tip: The month (1-12) of an instant.
+order: 4
+ns: DateTime
+member: month
+---
+
+The calendar month of the instant <code>dt</code> (UTC), <code>1</code>&ndash;<code>12</code>.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let m = DateTime.month(DateTime.from(2000, 3, 9, 0, 0, 0))   # 3
+  }
+}
+```

docs/language/datetime/datetime-second.md

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+---
+id: datetime-second
+name: DateTime.second
+category: datetime
+kind: namespace-method
+tokens: DateTime.second
+sig: DateTime.second(dt) -> int
+tip: The second of the minute (0-59) of an instant.
+order: 9
+ns: DateTime
+member: second
+---
+
+The second within the minute of the instant <code>dt</code> (UTC), <code>0</code>&ndash;<code>59</code>.
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let s = DateTime.second(DateTime.from(2000, 1, 1, 12, 30, 15))   # 15
+  }
+}
+```

docs/language/datetime/datetime-weekday.md

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+---
+id: datetime-weekday
+name: DateTime.weekday
+category: datetime
+kind: namespace-method
+tokens: DateTime.weekday
+sig: DateTime.weekday(dt) -> int
+tip: Day of the week, 0=Sunday..6=Saturday.
+order: 6
+ns: DateTime
+member: weekday
+---
+
+The day of the week of the instant <code>dt</code> (UTC), <code>0</code> (Sunday) through <code>6</code> (Saturday).
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let w = DateTime.weekday(DateTime.from(2000, 1, 1, 12, 0, 0))   # 6
+  }
+}
+```

docs/language/datetime/datetime-year.md

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+---
+id: datetime-year
+name: DateTime.year
+category: datetime
+kind: namespace-method
+tokens: DateTime.year
+sig: DateTime.year(dt) -> int
+tip: The calendar year of an instant.
+order: 3
+ns: DateTime
+member: year
+---
+
+The calendar year of the instant <code>dt</code> (UTC).
+
+```ludic
+program Demo {
+  handler Step phase Update {
+    let y = DateTime.year(DateTime.from(2000, 1, 1, 12, 30, 15))   # 2000
+  }
+}
+```